It is well known to allocate radio channels for speech users on an as-needed basis. There are several systems which serve as examples.
In a trunked dispatch radio system, it is well known to allocate radio channels upon the request of a subscriber. Basically, the subscriber radio, when not in active talking, monitors a control channel. When the subscriber wants to make a call to other subscribers, a push-to-talk (PTT) button is depressed which initiates a channel request sequence. A request data message is transmitted, and the subscriber radio reverts to a receive mode on that channel to listen for a channel grant. When a channel grant is received, the subscriber radio moves its synthesizer to the assigned frequency and a speech path is enabled. Speech can then be transmitted so long as the PTT is activated. A more detailed explanation of one such system is included in U.S. Pat. No. 4,821,310 to Lynk. Alternatively, a voice activated transmit (VOX) technique may be used.
Another such system is the so-called Extended-Time Division Multiple Access (E-TDMA) cellular radiotelephone system. In the E-TDMA system, cellular radio channels are allocated and de-allocated based on measured speech activity of the subscribers. When the subscriber stops talking for a period of time, the channel is relinquished for other subscribers. As soon as speech activity is detected, a channel is requested. Speech starts to use a particular voice channel as soon as a channel grant is received.
It is well known that the delay which occurs from the time the PTT is activated (or speech activity begins for VOX operated systems) until the time the speech channel is actually available for communications is quite deleterious to the communication. The reason for this is that some of the speech information is lost, and can never be recovered.
One solution to the lost speech problem is to buffer speech while the channel is being requested, and to replay the buffered speech after the voice channel actually becomes available. In this solution, the delay, which is often of an uncertain amount, is inserted into the voice channel. In a dispatch system, where acoustic feedback to the talker is often present, the delay inserted into voice channel can cause significant confusion. One well known example is in the public safety arena. At a fire site, one user may increase the volume of a radio so that it can be heard by a nearby users. If one of those nearby user has a portable radio and attempts to talk on the system, he will hear himself delayed in time. This scenario may cause significant confusion indeed.
Therefore, a need exists for a communication system which eliminates the loss of speech due to time delay incurred during channel set-up.